Arguably the main theme in the book is social prejudice and its detrimental effects. The story explores all types of social prejudice, most dramatically in the form of racism with Tom Robinson's trial. However, racism is only one aspect of the issue. Prejudice is shown to be pervasive and wide-ranging, certainly in a cramped, conservative little town like Maycomb. Society in this town is rigidly divided along lines of race, class and gender. The oppression of blacks - segregated, impoverished, and regarded as fair game for abuse and accusations - is obvious. There are only a few enlightened individuals, like Atticus, Miss Maudie, and Heck Tate, that are able and willing to see past a person's skin colour.
Class prejudice is also rife in this community. Aunt Alexandra does not allow Scout and Jem to mix with people of a lower social class, like the Cunninghams, to Scout's mystification and indignation. Most despised of all though are so-called 'poor white trash' like the Ewells. Although Bob Ewell is certainly villainous, we can assume that his treatment by society at large has probably helped to shape him into the mean and vile person that he is.
Gender is also a cause for prejudice. Women are seen to have sharply defined roles: they are expected to be good wives, mothers and housekeepers, to act and dress most genteelly, and little more. Those who refuse to be limited to such roles, like Miss Maudie, become the object of criticism - although Miss Maudie, at least, has a healthy disregard for what other people may think. She is more than happy to spend time in her garden rather than her house, and to wear overalls instead of dresses. Scout appears to be of a similar tomboyish inclination and rebels against her prim and proper aunt's efforts to mould her into a socially respectable model of femininity.
Finally, there is the kind of prejudice that exists against any one that who behaves differently from expected social norms in any way. This is true of Boo Radley. A shy and good-hearted man, he is misunderstood both by his own family as well as society at large, and so becomes a recluse, hiding away from the adult world altogether. He becomes the object of superstition, particularly for the children, but by the end his goodness is fully revealed to them and they see him for what he really is: one of the most decent, humane individuals in the entire town. They lose their former fear of him once they come to understand him. The book thus shows that prejudice can be overcome by employing understanding and empathy for others. This is the fundamental lesson that Atticus strives to teach his children.